1. Field of the Invention
This invention relates to a novel exo-type hydrolase (provisional name: exo-inulo-trio and tetrao-hydrase) which hydrolyzes a fructan specifically every 3-4 sugar units from the terminal fructose thereof, and to a method of manufacturing inulotriose and/or inulotetrose from a fructan using said hydrolase.
2. Prior Art
Inulotriose and inulotetrose, which respectively consist of 3 and 4 fructoses, are inulo-oligosaccharides with a high functionality (for example, bifidus factor, anticariogenicity, low calories, etc.), and they find wide application in the arts of foods and pharmaceuticals.
Conventionally, one method for preparing inulotriose and inulotetrose from a fructan has been to subject a fructan such as inulin to an acid hydrolysis reaction (JP62-232380).
According to this method, however, the main product was fructose (F.sub.1) and the yield of inulo-oligosaccharides, including inulotriose and inulotetrose, was extremely poor. In addition, colored substances were produced together with a by-product such as difructose anhydride, and thus the purification of inulotriose and inulotetrose was very difficult.
Another method has also been proposed wherein a fructan is hydrolyzed by means of an enzyme to produce inulotriose and inulotetrose (Barrie E. Norman and Birgitte Hojer-pedersen, Denpun Kagaku, 36, 103 (1989)). The enzyme used for this purpose, however, was only an end-type inulinase which hydrolyzes a fructan randomly. When therefore it hydrolyzed, for example, inulin, in addition to inulotriose and inulotetrose, other inulo-oligosaccharides consisting of various different numbers of fructoses were also produced such as inulobiose (F.sub.2), inulopentose (F.sub.5), and inulohexose (F.sub.6). Moreover, fructose itself was also produced as a by-product. Consequently the yield of inulotriose and inulotetrose was poor and the method was unsuitable for their manufacture.
We have studied on the methods by which inulotriose and inulotetrose could be manufactured from a fructan. As a result, we found a novel exo-type hydrolase (provisional name: exo-inulo-trio and tetrao-hydrase) which specifically hydrolyzes a fructan into oligosaccharides consisting of 3 to 4 fructoses.